glossal muscles, (2) the glosso-pharyngeal and vagus to the pharyngeal plexus to supply the constrictors, and (3) the facial and fifth to supply the fauces and palate, as indicated by their anatomical distribution.

The act of deglutition can be readily excited in an animal which is deprived of all the nerve centres down to the medulla oblongata, and may also be seen in those human monstrosities (anencephalous foetus) without the upper part of the brain being developed, but which can notwithstanding both suck and swallow.

The movements of the oesophagus are reflections from the central nervous system (medulla), both sets of impulses (possibly the afferent and certainly the efferent) passing along the branches of the vagus.

It would appear that the normal peristaltic movements of the œsophagus are always initiated by a pharyngeal spasm, and that they form an inseparable sequel to it. Thus the wave of contraction passes along the entire length of the oesophagus even when the bolus is stopped mechanically, and, on the other hand, a body introduced into the œsophagus without passing through. the pharynx excites no peristaltic wave, and remains motionless.

FIG. 54.

f

h

But it has been observed, in apparent contradiction to the foregoing statement, that the cesophagus when removed from the body, and therefore quite independent of the pharynx and its nervous connections, can be excited to move peristaltically. In this case the medulla or vagus can have no part in bringing about this wave of movement. To explain this discrepancy, it may be urged that the local nerve and muscle mechanism in the tissues of the oesophagus are capable by themselves of carrying out peristaltic contraction

Diagram of Wall of the Stomach, showing the relative thickness of the mucous membrane (a, b, c) and the transverse (e), oblique ) and longitudinal muscle fibres.

independently of the central nerve organs, but that this power is, under ordinary circumstances, held in check by the vagus. The inhibition is temporarily suspended as a sequence of pharyngeal spasm, and consequently a wave of peristaltic contraction is excited in the œsophageal muscles, either in response to the direct stimulus of a passing bolus, or as a result of impulses reflected along the vagus channels from the medulla.

Motion of the Stomach.-The stomach and greater part of the intestinal tract move freely within the abdomen, being covered by the smooth serous lining of that cavity, which also keeps in position, so as to restrict their movements, those parts, such as the duodenum, into which the ducts of large glands open. When the stomach is empty it hangs with the great curvature downward, and the muscular coats are quiescent. On being filled it is passively rotated on its long axis, so that the greater curvature is turned forward, here meeting with less resistance, and the lesser curvature is turned backward to its line of attachment. In the main, the motions of the stomach are peristaltic. They become very active about fifteen minutes after the introduction of food, and gradually become more and more energetic until the end of stomach digestion, which lasts about five hours.

The result of the peristaltic motion is to move the food, particularly the part next the gastric wall, along the great curvature toward the pylorus. A back current toward the cardiac extremity has been noticed running along the lesser curvature and the median axis of the food mass. At the same time a peculiar rotatory motion of the gastric wall takes place, similar to that of rolling a ball between the palms of the hands, so that the food is twisted in a given direction, and the deeper lying portion is brought into contact with the mucous membrane.

While the fundus keeps up considerable pressure on the contents of the stomach, the indistinct peristaltic action of the central parts is intensified, on nearing the pylorus, into a strong circular contraction, which proceeds as a definite wave toward the pyloric valve, through which it gradually forces the more or

less digested food.

At first only the fluid parts are allowed to pass, but toward the later stages of digestion the fatigued pyloric muscle admits solid masses into the duodenum.

Nerve Influence on Stomach Motions.-The stomach has nerve connections with the cerebro-spinal axis through the vagi, and the splanchnic branches of the sympathetic, and in the walls of the organ itself are numerous ganglion cells. The sympathetic connections do not seem to have any influence on the muscular coats, for neither their stimulation nor section has any marked effect on their movements. If the vagi be severed, stomach contractions still occur, but no form of local stimulation produces the normal gastric motions, even if the organ be quite full of food, therefore it would appear that the local nerve centres are not sufficient to excite the normal rhythmical muscular action. Moreover, stimulation of the cut vagi leading to the stomach causes active movements when the stomach is full. It is not merely the presence of food that produces the movements, as is shown by the fact that the motions increase as the contents of the stomach diminish, but conditions incidental to digestion (hyperæmia, etc.), probably also act as a stimulus.

Vomiting is the ejection of the contents of the stomach by means of a convulsive action of the respiratory and abdominal muscles associated with an abnormal contraction of the stomach wall, which aids in opening the cardiac orifice while it keeps the pylorus firmly closed.

The act of vomiting is commonly preceded by (1) a feeling of sickness or nausea, (2) a great secretion of saliva, (3) retching. The latter consists in a violent inspiratory effort, in the midst of which the root of the tongue and the larynx are raised and the rima glottidis suddenly closed so as to prevent air entering the windpipe. The inspiratory muscles still acting, and the pharynx and upper part of the œsophagus being held open, air is drawn into the gullet and dilates this tube nearly as far as the opening into the stomach. A contraction of the muscle fibres radiating from the oesophagus over the stomach then opens the cardiac orifice and allows some gas to escape.

Now the act of

vomiting is completed if at this moment-the mouth and pharynx being open, the larynx closed, the oesophagus on the stretch, the cardiac orifice relaxed, and the pylorus firmly closed-the expiratory muscles forcibly contract, and, pressing upon the abdominal cavity, give a sudden stroke to its contents so as to empty the stomach. The wall of the stomach also contracts evenly throughout, but not with any forcible anti-peristaltic action such as would greatly aid in the operation of rapidly ejecting the vomit. The chief object attained in the adult by the action of the muscular coat of the stomach seems to be the relaxation of the cardiac orifice. In children, when the fundus is little developed, and the fibres radiating over the stomach from the oesophagus are numerous and strong, the act of vomiting requires less effort on the part of the respiratory muscles; the frequent puking of suckling infants being accomplished by the gastric muscle alone. When the vomit is emitted, the hyoidean, laryngeal, and neck muscles relax, and the air is forcibly driven out of the partially distended lungs so as to clear away any remaining particles from the upper part of the air passages.

Vomiting is usually caused by irritation of the stomach itself, and may be induced by either mechanical, electrical, or chemical stimulation of the mucous membrane. In this way some emetics, such as mustard, sulphate of copper, etc., act. It may also be caused by intestinal irritation, as when a hernia is strangulated or the mucous membrane irritated by intestinal worms.

Gentle stimulation of the fauces and neighborhood of the root of the tongue commonly induces vomiting. In the early stages of pregnancy the unusual condition of the uterus causes frequent vomiting, which is known as "morning sickness." The irritation of a calculus passing through the ureter, or a gall stone impacted in the bile duct, usually excite vomiting. Injuries of the brain, and psychical impressions, particularly those excited by the sense of smell or unusual disturbance of equilibrium, may give rise to vomiting. Moreover, a number of medicaments, as apomorphin, emetin, etc., cause vomiting if introduced into the blood.

From the foregoing facts it appears that vomiting is a complex

and irregular muscular act, which may be induced by the stimulation of various parts of the internal surfaces of the body, particularly those which receive branches from the vagus nerve.

One would, therefore, be inclined to suppose that some afferent nerve channels exist in the vagus which bear impulses to a vomiting nerve centre and excite it, so as to cause it to send forth peculiar and irregular impulses to the respiratory, gastric, and other muscles, and give rise to their characteristic spasm.

In short, it would seem to be a reflex act, the afferent impulses of which pass to the medulla oblongata by the vagus, and the efferent impulses are conveyed by the ordinary spinal nerves to the respiratory muscles by the vagus to the pharyngeal, laryngeal and gastric muscles, and by the fifth, seventh and ninth nerves to the palatine, facial and hyoidean muscles. This vomiting nerve centre must lie in the medulla, in very close relationship to the respiratory centre, with which it nearly corresponds. This centre may bring about the whole sequence of events known as vomiting, when stimulated either directly by poisons contained in the blood, indirectly through the vagus, or even from the higher centres by emotions or ideas. Section of the vagi renders vomiting impossible, as it cuts off both the commonest source of stimulus going to the centre, and also the important efferent impulses which cause the muscle coat of the stomach to contract and to open the cardiac orifice.

Movements of the Intestines.-The muscular coats are somewhat differently arranged in the small and the large intestines, but have the same general relation to each other, viz., a thin longitudinal layer lying externally, next the serous membrane, and a layer of circular fibres considerably thicker lying internally under the mucous membrane. In the large intestine the external longitudinal fibres are collected into three bands placed at equal distances one from another, which, being rather shorter than the remainder of the intestine, throw the intermediate part into a series of pouches.

It is in the small intestine that peristaltic motion of the most typical kind occurs. A wave of contraction passes from the